Thermoregulation under semi-natural conditions in two species of African barbets (Piciformes: Lybiidae)

Abstract

Torpor has been documented in a number of avian orders, but our understanding of the phylogenetic distribution of this phenomenon is far from complete, and several groups have yet to be investigated. We examined patterns of rest- and active-phase thermoregulation in Acacia Pied Barbets Lybius leucomelas and Crested Barbets Trachyphonus vaillantii held in outdoor aviaries, using surgically-implanted miniature data loggers to record body temperature (Tb). Both species exhibited large circadian rhythms of Tb, with maximum active-phase and minimum rest-phase Tb (ρTbmin) values of 42.3 ± 0.1 °C and 37.7 ± 0.6 °C, respectively, in Acacia Pied Barbets (n = 3) and 42.5 ± 0.4 °C and 37.0 ± 0.6 °C, respectively, in Crested Barbets (n = 5). Neither species exhibited any indication of torpor, despite being subjected to a food restriction treatment that is known to elicit torpor in other taxa. Instead, restricted feeding resulted in small but significant increases in the amplitude of circadian Tb rhythms, with ρTbmin values 0.3-0.6 °C lower than during ad libitum food availability. The responses of barbets maintained in aviaries may not, however, be representative of those of free-ranging individuals. Body temperature traces we obtained from Acacia Pied Barbets in a field laboratory in the Kalahari Desert reveal reductions in Tb to below 32 °C during the rest-phase, suggesting that in natural habitats this species may routinely exhibit facultative hypothermia. The amplitudes of circadian Tb rhythms we observed under seminatural conditions in both species are significantly greater than expected on the basis of an allometric equation based largely on Northern Hemisphere data, but are similar to those observed in other southern African species.